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Colorado CDL Combination Vehicles study materials
Colorado CDL Combination Vehicles study resources

Colorado CDL Combination Vehicles: Air Brakes and Combination Vehicle Braking Systems

Colorado CDL Combination Vehicles · Work through your exam, one topic at a time.

All guide sections

Air Brakes and Combination Vehicle Braking Systems

What this domain tests

This domain covers the air-brake system on a combination: supply and service lines, tractor protection valve, trailer spring brakes, low-pressure warning, leak tests and brake adjustment. You must know what happens when pressure falls and how to test both tractor and trailer brakes.

Core rules

  • The compressor charges reservoirs; the governor controls cut-in and cut-out. The tractor protection valve protects the tractor if the trailer separates or leaks. It closes automatically at about 20-45 psi.
  • A low-pressure warning must activate before pressure falls below 60 psi. Spring brakes apply automatically at roughly 20-45 psi.
  • With a charged system, engine off and parking brake released, pressure loss must not exceed 2 psi in one minute for a single vehicle or 3 psi for a combination. With the foot brake applied and held, loss must not exceed 3 psi in one minute.
  • Test trailer service brakes separately: chock wheels, release the trailer parking brake, apply the trailer hand valve or service brake, and try to pull forward. The trailer brakes should hold.
  • Check slack adjusters and pushrod travel. If a pushrod stroke is beyond the limit, the brake is out of adjustment and the vehicle should be placed out of service.
  • On long grades, use a low gear and engine braking; do not ride the service brake and overheat the drums.

Worked example

A combination is charged to 100 psi. Engine off, parking brake released, one minute later the gauge reads 97 psi. That is a 3 psi drop, which is at the limit for a combination and acceptable. Next, hold the foot brake; after one minute the gauge reads 92 psi. A 5 psi drop exceeds the 3 psi applied-brake limit, so the vehicle fails the leak test and must be repaired before driving.

Traps

Using the single-vehicle leak limit on a combination, skipping the trailer brake test, ignoring a slow pressure drop, and confusing the low-pressure warning with the spring-brake application point all cost marks. Never fan the brakes to cool them.